Acute Promyelocytic Leukemia (APL) is characterized by a maturational arrest of bone marrow progenitors at the level of the promyelocyte. APL is unique amongst the leukemias in that retinoic acid induces differentiation of the blasts, and induces remissions in patients. We have been exploring the molecular mechanism underlying APL, with the long-term goal of developing strategies of differentiation therapy for other malignancies. We were the first group to clone the t(5;17)( q35;q21) variant of APL. This translocation generates an in-frame fusion of the N-terminal 117 amino acids of nucleophosmin (NPM) with the C-terminal 403 amino acids of the retinoic acid receptor alpha (RARa). NPM-RAR serves as a unique "experiment of nature" with which to identify the common molecular mechanisms underlying APL. We have found that NPM- RAR expression can block myeloid differentiation, both in cell models and in transgenic animals. Others have shown that a truncated RARa that harbors just the C-terminal 403 amino acids of RARa is insufficient to cause APL. Indeed, our preliminary studies suggest that NPM-RAR may bind proteins that do not interact with RARa. In this proposal we will determine the contribution of NPM to the function of the NPM-RAR protein. We will test the hypothesis that the NPM-sequences within NPM-RAR are essential for its ability to generate the APL phenotype. Our Specific Aims are 1) to determine whether the NPM-sequences serves as a protein-interaction domain for NPM-RAR;2) to determine the mechanism whereby NPM alters RARa transcriptional activity;3) to determine whether NPM-RAR inhibits wild-type NPM function. These studies will identify the role of the RARa fusion partner in t(5;17) APL, and will help identify common pathways that underlie the APL phenotype. Acute Myeloid Leukemia (AML) is a significant public health issue. Understanding the detailed mechanism whereby expression of the NPM-RAR fusion protein leads to APL may lead to deeper understanding of the success of differentiation therapy, and generate novel therapeutic strategies that can be applied to other leukemias. In addition, the recent finding of NPM-rearrangements in the majority of normal-cytogenetic AML patients increases the significance of these studies of mutated NPM beyond the variant APL in which NPM- RAR is found.